The successful implementation of a lightwave communication system requires the manufacture of high quality lightguide fibers having mechanical properties sufficient to withstand stresses to which they are subjected. Typically, the fiber has an outside diameter of 0.125 mm and is drawn from a glass preform having an outer diameter of 17 mm. Each fiber must be capable of withstanding, over its entire length, a maximum stress level which the fiber will encounter during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundred circuits.
The failure of lightguide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength from that of the pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress. Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Long lengths of lightguide fibers have considerable potential strength but the strength is realized only if the fiber is protected with a layer of a coating material such as a polymer, for example, soon after it has been drawn from a preform. This coating serves to prevent airborne particles from impinging upon and adhering to the surface of the drawn pristine fiber which would serve to weaken it. Also, the coating shields the fiber from surface abrasion, which could be inflicted by subsequent manufacturing processes and handling during installation, provides protection from corrosive environments and spaces the fibers within cable structures. Thus, it is common to apply a coating material to the outer surface of the fiber. Typical coating cups used to apply such coating materials are shown in U.S. Pat. Nos. 4,409,263 and 4,208,200. Accordingly, the drawn fiber passes through the coating cup after being drawn from the preform and prior to reeling.
It is necessary that the fiber, which is heated to approximately 2000.degree. C. during the drawing operation, be cooled to below 80.degree. C. prior to applying the coating. The cooling of the fiber is necessary in order to avoid heating and charring of the coating material resulting in an unacceptable coated fiber.
One technique for cooling the fiber is to decrease the drawing speed to permit the fiber to sufficiently cool prior to entering the coating material. Such a method can provide an acceptable coating but undesirably results in low quantity of product for a given time period.
An additional technique is to provide a substantial distance between the drawing furnace and the coating cup. This distance provides sufficient time for the fiber to cool before coating. Unfortunately, the equipment to maintain such distances is expensive and many factories do not have the necessary vertical height (e.g., 10 meters) available.
A third method for cooling the drawn fiber is described in U.S. Pat. No. 4,208,200. That patent depicts a vessel containing a liquid coating material interposed between a furnace and the coating cup which is used to precoat and cool the fiber. The fiber passes through the liquid and through a wiper in the bottom of the cup. The high heat transfer coefficient of the material provides cooling of the fiber which permits a closer physical relation between the coating cup and the furnace and/or a higher drawing speed. However, as indicated above, physical contact with the pristine fiber with an apparatus such as a wiper can deleteriously weaken the coated fiber.
Accordingly, there is a need for a technique which rapidly cools the drawn fiber while avoiding any physical contact therewith.